1. Field of the Invention
The present invention relates to capacitors used in diverse electronic devices, and more particularly to chip-type solid electrolytic capacitors which employ conductive polymer as their solid electrolyte and are designed for surface mounting.
2. Background Art
The trend towards higher frequencies being used in electronic devices has led to growing demand for capacitors, among electronic components, that have better impedance characteristics in the higher frequency range. To meet this need, various solid electrolytic capacitors using conductive polymer with high electric conductivity as their solid electrolyte have been examined.
There is also a strong demand for smaller solid electrolytic capacitors with greater capacitance for use in peripheral circuits of the CPUs of personal computers. In parallel with higher frequencies, reduction of equivalent series resistance (ESR) is also required. Still more, as there are also strong demands for noise removal and good transient response, reduction of equivalent series inductance (ESL) of solid electrolytic capacitors is requested. Studies to meet these demands are being undertaken.
FIG. 6 is a perspective view of one of the conventional chip-type solid electrolytic capacitors disclosed in Japanese Patent Unexamined Publication No. H6-120088. FIG. 7 is a perspective view of the internal structure of this chip-type solid electrolytic capacitor. The conventional chip-type solid electrolytic capacitor has a structure in which two sheets of capacitor element 20 using conductive polymer as solid electrolyte are overlaid. This capacitor element 20 has anode 21, cathode 22, and insulating section 23, and two sheets of capacitor element 20 are overlaid such that anodes 21 protrude in opposite directions.
The conventional chip-type solid electrolytic capacitor further includes anode lead terminal 24 whose one end is coupled to anode 21 of capacitor element 20, cathode lead terminal 25 whose one end is coupled to cathode 22, and coating resin 26 molded so as to cover capacitor element 20. A pair of anode lead terminals 24 are disposed opposing each other and a pair of cathode lead terminals 25 are disposed opposing each other on the side face and bottom face of the solid electrolytic capacitor molded and covered with coating resin 26. This configures a 4-terminal solid electrolytic capacitor.
The conventional chip-type solid electrolytic capacitor as configured above has good high-frequency characteristics and noise absorbability, combined with low ESL.
However, the above conventional chip-type solid electrolytic capacitor can only suppress ESL to about 500 pH (picohenry) at the most compared to general two-terminal chip-type solid electrolytic capacitors in which one or multiple sheets of capacitor element 20 are laminated and molded with the coating resin, and anode/cathode terminals are led out. In the current market, ESL of 200 pH or below is demanded. Accordingly, the chip-type solid electrolytic capacitor shown in FIGS. 6 and 7 still does not meet this high requirement, and thus further reduction of ESL remains a pending task.